Wednesday, November 9, 2011

Five crazy weather events you could be looking at right now

1) The extremely deep low in the Bering Sea
An unusually deep low pressure center moved through the Bering Sea last night and is now moving northward through the Chukchi Sea in western Alaska. At its peak intensity, the low was down to 943 mb (or so we assume). It was still at 946 mb, at least according to this morning's 12Z surface analysis from the NAM-WRF model over Alaska:

Hurricane-force winds are being reported with this storm, which is unusually far north to be of such a deep magnitude. Here's the Alaska surface observations from 06Z early this morning. Look at the wind observations in the Bering Sea. Anywhere you see a triangle-shaped flag on the wind barb, that means winds over 50 knots were reported. Amazing.

NOAA runs an ocean wave modeling system called WAVEWATCH III that forecasts wave heights over areas in time. Here's their 12Z analysis of wave heights across the Alaska region.

Within that orange area in the center of the Bering Sea, they're showing waves as high as 9-10 meters--that's 30 feet or more. In fact, some reports have come in from buoys and ships reporting 40-foot waves with a wave period of only 14 seconds. That means that if you were on a boat, every 14 seconds you would fall 40 feet, then rise 40 feet on the waves. Spectacular. You can also see on this analysis map the grayed-out area to the north. This represents areas that are already completely iced over for the winter. That area is still pretty far north, so villages on the western Alaska Coast that would normally be protected from these waves by sea ice don't have that protection yet. Major coastal flooding and storm surge is expected.

2) The anomalously deep cut-off trough forecast over the eastern Pacific
Anyone who was planning on heading down to southern California this weekend hoping to escape the winter weather may be in for a bit of a shock. The same highly-amplified upper-air pattern that helped get that Alaskastorm going is going to help spin off a deep, cut-off, upper-level low that looks like it will hover off the coast of California for the next few days. Here's the 36-hour forecast for 500mb heights and temperature on Thursday evening from the UW-WRF 36km model:

You can see that this low is paired with a very strong ridge of higher heights over the central Pacific and that the main jet stream (where the iso-height lines are packed closely together) is much further north. This means that the low center is going to hang around for a while. Notice that with counterclockwise flow around that low height center it's directing onshore flow right into California in addition to bringing in colder air aloft. This combination is going to help bring cloudiness and rain showers to that area over the next few days.

We can compare this trough and ridge to the climatological normal values for 500mb heights to see just how unusual of a pattern this is. Here's the CPC's 3-day forecast for 500mb height anomalies over this area:

Anywhere surrounded by dashed red lines is anomalously high and anywhere surrounded by dashed blue lines is anomalously low. You can see that, particularly with respect to that ridge in the central Pacific, this pattern in somewhat unusual. It will definitely make for some interesting weather.

3) First snowfall across parts of the plains and the upper midwest
With the jet stream shifted so far north due to this huge ridge over the northern Pacific, it's getting to be kind of hard to make other shortwaves move along and propagate away. The same surface low that helped create severe thunderstorms in Oklahoma the other day has been slowly moving northeastward and getting better organized. It's currently located over western Michigan:

Note the very tight isobar packing on the western side of the low. This implies strong northwesterly winds in that area, helping to advect in some very cold air. Behind this low, temperatures look cold enough to support snow. Here's the RUC analysis of 1000-500mb thickness (an indication of how cold the lowest part of the atmosphere happens to be) for 18Z today. Anything north of the solid blue line should be cold enough to support snow:

As the low moves eastward and the cold front continues to swing south, areas in eastern Illinois and Wisconsin and into Michigan should see temperatures start to fall over the next few hours, eventually to a point where any precipitation that falls could be snow. Combined with the high winds behind the low pressure center, we can see why winter weather advisories have been posted for parts of Wisconsin.

4) Sub-tropical and now Tropical Storm Sean

Take a look again at the surface analysis that I showed two images ago. Notice the rather deep low analyzed off the coast of the southeastern US? That's tropical storm Sean (formerly sub-tropical storm Sean), a late-season tropical storm that should stay well-away from land. On visible satellite, we can see that it has a good spiral shape, but it's not very symmetrical--there are bands within the storm with little cloud content and no well-defined eye. These are all evidence that Sean isn't particularly strong--but still a tropical storm.

The National Hurricane Center official forecast has Sean staying a tropical storm for a while, feeding off the warm waters in the Gulf Stream. There is some evidence in the dynamical models and satellite imagery to suggest that Sean may strengthen to a weak category one hurricane, though, so I wouldn't be surprised to see it upgraded sometime soon. However, they don't project it to hit mainland North America at all. It may clip Bermuda, but otherwise it's staying well out to sea.

5) The "tropical storm" that formed in the Mediterranean Sea

If you really want an unusual cyclone, for the last several days a warm-core low developed and lingered over the western Mediterranean Sea. The CIMSS satellite blog had several loops of satellite images as the cyclone reached its peak intensity. Based on the warm-core structure and that the peak winds reached tropical storm strength, this was considered to be a tropical cyclone. In the Mediterranean. Strange...
As you can see, there's been a lot going on in the weather recently. Plenty to keep any meteorologist, from amateur to professional, occupied for quite some time...

1 comment:

This is a great post Luke! Very informative. There has been some crazy weather this year, with April being especially insane for tornadoes. We are undergoing a pattern change here in the northwest now, so we'll see what the next few weeks bring.

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
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